This invention relates to an electron beam aperture device for use in a scanning electron microscope or other such electron beam apparatus.
In a scanning electron microscope or X-ray microanalyzer, analysis of the elements contained in the specimen under examination is carried out by measuring the energy (or wavelength) of the characteristic X-rays which are generated when the electron beam irradiates said specimen. However, in the type of apparatus in question, stray X-rays emanating from various mechanical appurtenances in and around the beam path, especially any aperture mechanism located in the vicinity of the specimen, are detected by the incorporated X-ray spectrometer. This is a cause of signal background noise which is especially pronounced if the detector happens to be an energy dispersive type X-ray spectrometer.
FIG. 1 is a schematic drawing for explaining the drawbacks inherent in the prior art. Referring to the figure, an aperture disk 1 is provided with a thru-hole 2 through which an electron beam 3 passes. The disk 1 is made, as a general rule, of tantalum or platinum and is machined to thickness of approximately 0.6 mm. Some of the X-rays (stray X-rays) 4 emanating from the sidewalls of the thru-hole 2, resultant upon electron beam impingement thereon, are directed towards and strike up against the specimen 5 and its holder 6. This, in turn, results in the emanation of further X-rays 4' which are detected by an energy dispersive type X-ray spectrometer 7 and which are different from the X-rays 8 (also detected by spectrometer 7) emanating from the specimen 5, resultant upon direct electron beam irradiation thereon. Accordingly, the signal attributable to the X-rays 4' is mixed with the "true" signal attributable to X-rays 8 and, because of this, the presence of background noise is unavoidable. As a means of alleviating this problem, that is, obstructing the passage of stray X-rays 4, an additional aperture plate 9, having a thru-hole diameter somewhat larger than that of the aperture disk 1, has been incorporated and arranged below this disk aperture 1. However, said means is not sufficiently effective to be considered satisfactory, not to mention the fact that aligning the two aperture centers is troublesome to say the least.